The present invention relates to a magnetic recording medium having a lubricant layer containing an organic fluorine compound over a magnetic layer. More particularly, the present invention relates to a metal thin film type magnetic recording medium which is excellent in running properties, durability and shelf stability.
To improve magnetic properties required for high-density magnetic recording in high-definition broadcasting, digital broadcasting, etc., ferromagnetic metal thin film type magnetic recording mediums formed by vapor deposition or the like have been demanded in place of the conventional coating type magnetic recording mediums. However, in a ferromagnetic metal thin film type magnetic recording medium, a metal layer, which serves as a recording layer, is formed on the surface of the magnetic recording medium, and the metal layer is usually protected with only an extremely thin oxide layer formed on the metal surface. Thus, the ferromagnetic metal thin film type magnetic recording mediums have been demanded to improve markedly in running properties, durability, running durability in repeated use, shelf stability, etc.
Under these circumstances, it has been proposed to coat the surface of the thin film metal layer with various lubricants.
For example, Japanese Patent Application Laid-Open (KOKAI) No. 62-256218 (1987) proposes using a carboxylic acid perfluoroalkyl ester RCOO(CH.sub.2).sub.j C.sub.k F.sub.2k+1. Japanese Patent Application Laid-Open (KOKAI) No. 62-236120 (U.S. Pat. No. 4,735,848) discloses a magnetic recording medium provided with a lubricant layer containing a carboxylic acid perfluoroalkyl ester and an extreme pressure agent. Japanese Patent Application Laid-Open (KOKAI) No. 62-236118 (U.S. Pat. No. 4,735,848) discloses a magnetic recording medium provided with a lubricant layer consisting essentially of a carboxylic acid perfluoroalkyl ester and a perfluoro polyether. Japanese Patent Application Laid-Open (KOKAI) No. 62-236119 (U.S. Pat. No. 4,735,848) discloses a magnetic recording medium provided with a lubricant layer consisting essentially of a carboxylic acid perfluoroalkyl ester and a rust preventive. However, all of the carboxylic acid perfluoroalkyl esters, which are principal components of the above-described lubricant layers, are composed of a carboxylic acid of a hydrocarbon system unsaturated fatty acid, and hence readily decomposable. Accordingly, these lubricant layers are unstable under sever conditions, such as high-temperature and high-humidity conditions, and suffer from a large variation of the coefficient of friction.
Japanese Patent Application Laid-Open (KOKAI) No. 62-266728 (1987) discloses a magnetic recording medium that uses a carboxylic acid perfluoroalkoxyalkyl ester RCOO(CH.sub.2).sub.n OC.sub.m F.sub.2m+1. The disclosed lubricant is, however, hydrolytic because it has a hydrocarbon group and hence decomposed during storage for a long time, resulting in a failure to exhibit the required lubricating function. Moreover, the decomposition product may impair running properties and durability. Japanese Patent Application Laid-Open (KOKAI) No. 63-9011 (1988) proposes using a perfluorocarboxylic acid perfluoroalkyl ester C.sub.n F.sub.2n+1 COO(CH.sub.2).sub.m C.sub.k F.sub.2k+1. However, the lubricant is unsatisfactory from the viewpoint of the variation of the coefficient of friction and the still durability.
Japanese Patent Application Laid-Open (KOKAI) No. 63-298709 (1988) discloses a technique in which C.sub.n F.sub.2n+1 (CH.sub.2).sub.m X (X is OH, COOH, or COOR) is mixed with at least one selected from the group consisting of a higher fatty acid, an ester, and an alcohol. Japanese Patent Application Laid-Open (KOKAI) No. 03-241523 (1991) proposes using RfCOOR (Rf is an alkyl group a part of which is fluorine, and R is an alkyl group having not more than 5 carbon atoms). However, these lubricants are unsatisfactory with regard to running properties at low temperature, suffer from a high coefficient of friction and are lacking in anti-corrosion properties. Accordingly, these conventional magnetic recording layers gather rust during storage for a long time, resulting in the running properties and durability being degraded.
Japanese Patent Application Post-Examination Publication No. 03-19602 (1991) discloses a ferromagnetic metal thin film type magnetic recording medium which uses as a lubricant a fluorine-containing surface-active agent, or a fluorine-containing hydrocarbon having a group, e.g., SO.sub.3 Na, SO.sub.3 K, SO.sub.3 H, etc., more specifically, C.sub.8 F.sub.17 SO.sub.2 F, or perfluoroalkylpolyoxy ethylene having OH, SO.sub.3 M, or RfCONH(CH.sub.2).sub.3 (RF is a perfluoroalkyl group) at a terminal thereof, i.e., compounds having a terminal betaine structure. With these fluorine compounds, excellent running properties can be obtained, but still durability and shelf stability are not satisfactory. Moreover, these fluorine compounds are insoluble in a hydrocarbon solvent and therefore need a special solvent, e.g., a fluorine-containing solvent, for coating.
Japanese Patent Application Laid-Open (KOKAI) No. 64-72313 (1989) discloses a technique in which a monomolecular film having the structure of Rf--Rh--X, i.e., perfluoroalkyl group--alkyl group--polar group, is formed on a magnetic layer of a ferromagnetic metal thin film. With this technique, the .mu. value is improved, but the formation of a monomolecular film by Langmuir Blodgett (LB) process is inferior in productivity and hence impractical. Further, the magnetic recording medium is inferior in still durability and shelf stability, and the solubility of the lubricant in a hydrocarbon solvent is not insufficient.
Japanese Patent Appl icat ion Laid-Open (KOKAI) No. 58-100228 (1983) discloses a lubricant with two chains in which a perfluoroalkyl group and an alkyl group are bonded together, and mentions that the double-chain lubricant (C.sub.3 F.sub.7 CH.sub.2 CO.sub.2 --) is applied to a coating type magnetic recording medium. However, it is impossible to expect such a lubricant with a short chain length to satisfactorily lower the coefficient of friction when it is used as a protective layer for a magnetic recording layer of a ferromagnetic metal thin film. In addition, when applied to a ferromagnetic metal thin film type magnetic recording medium, the lubricant suffers from the disadvantages that the corrosiveness is undesirably high, and the coefficient of friction is high. Moreover, it is necessary in order to coat the lubricant to use a coating solution prepared by dissolving the lubricant in a fluorine-containing solvent. Further, since the lubricant itself readily hydrolyzes, the magnetic recording medium using this lubricant cannot be stored for a long period of time.
Japanese Patent Application Laid-Open (KOKAI) No. 62-92227 (1987) discloses a ferromagnetic metal thin film type magnetic recording medium which uses an ammonium salt of Rf(CH.sub.2).sub.n COO as a fluorine compound lubricant or uses as a lubricant a compound having a fluoroalkyl group having not less than 3 carbon atoms, an aliphatic alkyl group having not less than 8 carbon atoms, and an OH group. These compounds can be dissolved in isopropanol to prepare a coating solution, and thus are advanced lubricants. However, they are still unsatisfactory in terms of such characteristics as .mu. value, still durability, and shelf stability, and cannot exhibit excellent characteristics under a wide variety of environmental conditions, particularly, over a wide temperature range of from high temperature to low temperature, or a wide humidity range of from high humidity to low humidity.
It has also been proposed to use perfluoroalkyl polyethers and derivatives thereof as lubricants in addition to the above-described lubricants [for example, see Japanese Patent Application Laid-Open (KOKAI) Nos. 60-61918 (1985), 61-107528 (1986), U.S. Pat. No. 3,778,308, Japanese Patent Application Post-Examination Publication No. 60-10368 (1985), U.S. Pat. No. 4,897,211, etc.]. However, these lubricants of fluorine-containing organic compounds suffer from an undesirably high coefficient of friction occurring between the magnetic layer and a sliding member of a magnetic recording and reproducing apparatus during running at low speed and therefore cannot impart satisfactory running properties to the magnetic layer. The lubricants are also unsatisfactory with regard to shelf stability; the magnetic layer may peel off due to corrosion by an acidic gas, e.g., sulfurous acid gas. Accordingly, no practicable magnetic recording medium can be obtained with these lubricants.
Fatty acids and fatty acid esters [for example, see Japanese Patent Application Post-Examination Publication No. 51-39081 (1976) and Japanese Patent Application Laid-Open (KOKAI) Nos. 56-80828 (1981) and 61-24017 (1986)] suffer from an undesirably high coefficient of friction occurring between the magnetic layer and a magnetic head during running at high speed and are also unsatisfactory with regard to the resistance to corrosion by an acidic gas, e.g., sulfurous acid gas.
Further, most of the above-described lubricants are inferior in solubility in ordinary hydrocarbon-containing organic solvents and soluble in only fluorine-containing organic solvents. Therefore, a fluorine-containing organic solvent must be used to coat a conventional lubricant on a magnetic recording layer as a protective layer. Fluorine-containing organic solvents are costly and causative agents that destroy the ozone layer. Thus, fluorine-containing organic solvents have serious problems from the viewpoint of protecting natural environment, and also suffer from the problem that they have an adverse effect on the manufacturing system and deteriorate the working atmosphere.
The present invention provides a lubricant which is soluble in an ordinary hydrocarbon-containing organic solvent, thereby solving the problems arising from the use of a fluorine-containing solvent, and which imparts running properties, durability and shelf stability to a ferromagnetic metal thin film type magnetic recording medium obtained by vapor deposition, sputtering or the like, whereby the recording density is markedly improved.
It is an object of the present invention to provide a magnetic recording medium, for example, a high-density recording medium, particularly, a metal thin film type magnetic recording medium capable of markedly improving the recording density, which is given excellent running properties, durability, repeated running durability, and shelf stability.
It is another object of the present invention to provide a magnetic recording medium for a cam coder useful for outdoor use.
It is still another object of the present invention to provide a magnetic recording medium capable of exhibiting satisfactory running properties and durability under various environmental conditions, i.e., over a wide temperature range and a wide humidity range.
It is a further object of the present invention to provide a method of producing a magnetic recording medium using a lubricant which is soluble in a hydrocarbon-containing organic solvent so that it is unnecessary to use a fluorine-containing organic solvent, and thus the production method is free from environmental pollution.